Gearbox and electric assist device

Abstract

The invention provides a gear box capable of effectively suppressing deformation during use and an electric auxiliary device provided with the gear box. The gear box (16) is provided with a wheel housing part (26) and a worm housing part (27). The wheel housing section (26) has a wheel cylinder section (28), an annular wheel bottom section (29), and a wheel fitting cylinder section (30) into which a wheel bearing is fitted. The worm housing section (27) has worm fitting cylinder sections (32, 33) in which a worm bearing or a bearing seat for holding the worm bearing is fitted, the worm fitting cylinder sections (32, 33) being provided at two locations separated in the axial direction. And a one-side rib (37) that protrudes from a side surface on one side in the axial direction of the wheel bottom (29) and connects the wheel fitting cylinder section (30) and the worm fitting cylinder section (32) on one side in the axial direction.

Description

[0001] This application is a divisional application; the application number of the parent application is "2022800617447", and the name of the invention is "Gearbox and electric auxiliary device". Technical Field

[0002] The present disclosure relates to an electric assist device and a gear box constituting the electric assist device. Background Art

[0003] As a device for reducing the force required for operating a steering wheel when a steering angle is applied to the steering wheels of a vehicle, an electric power steering device including an electric assist device for applying auxiliary power to a steering force transmission path is widely used.

[0004] The structure of the electric power steering device is roughly classified according to the installation position of the electric power assist device. Specifically, various structures have been proposed, such as a column assist type in which auxiliary power is applied to a steering shaft rotatably supported on the inner side of a steering column, a pinion assist type in which auxiliary power is applied to a pinion shaft as an input shaft of a steering gear unit, and a double pinion type in which a steering gear unit is provided with a pinion shaft different from the pinion shaft as an input shaft and auxiliary power is applied to the different pinion shaft.

[0005] In any structure, the electric assist device includes an electric motor as a power source and a speed reducer, and the auxiliary power of the electric motor is applied to a shaft member that rotates or moves linearly by the operation of the steering wheel through the speed reducer. As such a speed reducer, a worm speed reducer is widely used.

[0006] The worm reducer includes a worm wheel and a worm driven to rotate by an electric motor. The worm wheel has gear teeth on the outer peripheral surface. The worm has worm teeth meshing with the gear teeth on the outer peripheral surface. The worm wheel and the worm are housed on the inner side of a gear box. The gear box includes a wheel housing portion for housing the worm wheel on the inner side and a worm housing portion for housing the worm on the inner side. The worm housing portion is arranged in a twisted position relative to the wheel housing portion, and is connected to a circumferential portion of the radially outer end of the wheel housing portion. The worm wheel is rotatably supported on the inner side of the wheel housing portion via a wheel bearing. The worm is rotatably supported on the inner side of the worm housing portion via a worm bearing.

[0007] The electric motor is supported at the end of one axial side of the worm housing portion, and drives the worm to rotate when in use. That is, the electric assist device increases the torque by transmitting the auxiliary power generated by the electric motor to the worm wheel via the worm, and then applies it to the shaft component that rotates or moves linearly by the operation of the steering wheel, thereby reducing the force required by the driver to operate the steering wheel.

[0008] In the electric assist device, when the assist power is generated, meshing reaction force is applied to the worm and the worm wheel from the meshing portion between the worm teeth and the wheel teeth. Moreover, these meshing reaction forces are transmitted to the worm housing portion via a worm bearing for rotatably supporting the worm on the inner side of the worm housing portion, and are transmitted to the wheel housing portion via a wheel bearing for rotatably supporting the worm wheel on the inner side of the wheel housing portion. As a result, a force in a direction of separating the worm housing portion and the wheel housing portion from each other and a force in a direction of twisting the worm housing portion and the wheel housing portion are applied to the gear box.

[0009] On the other hand, International Publication No. 2019 / 087945 describes a structure in which a rib is provided on the gear case and the worm housing portion and the wheel housing portion are connected by the rib.

[0010] Prior art literature

[0011] Patent Literature

[0012] Patent Document 1: International Publication No. 2019 / 087945 Summary of the invention

[0013] Problems to be solved by the invention

[0014] In the structure described in International Publication No. 2019 / 087945, ribs provided on the gear box connect the worm housing portion and the wheel housing portion, thereby preventing the gear box from being deformed by forces acting on the gear box in a direction of separating the worm housing portion and the wheel housing portion and twisting them against each other when an auxiliary torque is generated.

[0015] However, in the structure described in International Publication No. 2019 / 087945, when manufacturing the gearbox by casting or injection molding, the position of the rib is determined in consideration of improving the fluidity of the material. Therefore, there is room for improvement in terms of more effectively suppressing the deformation of the gearbox by providing the ribs on the gearbox.

[0016] An object of the present disclosure is to provide a gear box capable of effectively suppressing deformation during use, and an electric assist device including the gear box.

[0017] Solutions to Solve Problems

[0018] A gear box according to an aspect of the present disclosure includes a wheel housing portion that houses a worm wheel therein, and a worm housing portion that houses a worm therein.

[0019] The above-mentioned wheel storing portion comprises: a wheel cylinder portion, which is arranged around the above-mentioned worm wheel; an annular wheel bottom portion, which extends radially inward from the end portion on one axial side of the above-mentioned wheel storing portion in the above-mentioned wheel cylinder portion; and a wheel interlocking cylinder portion, which is connected to the radially inner end portion of the above-mentioned wheel bottom portion and has a wheel bearing embedded therein for supporting the above-mentioned worm wheel.

[0020] The worm housing portion is configured as a cylinder with an open end portion on at least one axial side of the worm housing portion, and its own central axis is arranged at a position twisted relative to the central axis of the wheel housing portion, and is connected to a portion of the circumferential direction of the radially outer end portion of the wheel housing portion. The worm housing portion has a worm fitting cylinder portion at two locations separated along the axial direction of the worm housing portion, and a worm bearing for supporting the worm or a bearing seat for holding the worm bearing is embedded in the worm fitting cylinder portion.

[0021] A gear box of one embodiment of the present invention comprises a side rib which protrudes from the side surface on one axial side of the wheel housing portion in the wheel bottom and connects the wheel housing portion and the worm housing portion in the two worm housing portions.

[0022] In the gear box according to one aspect of the present disclosure, the one side rib extends in a radial direction centered on a central axis of the wheel bottom portion.

[0023] In a gear box according to one aspect of the present disclosure, the one-side rib is arranged on a straight line connecting the center axis of the wheel bottom and the center point of the worm fitting cylinder on the axial one side of the worm housing portion when viewed from the axial one side of the wheel housing portion.

[0024] In a gear box of one embodiment of the present invention, there is a rib on the other side, which protrudes from the side surface on one axial side of the wheel housing portion in the wheel bottom, and connects the wheel fitting cylinder portion and the worm fitting cylinder portion on the other axial side of the worm housing portion in the two parts.

[0025] In the gear box according to one aspect of the present disclosure, the other side rib extends in a radial direction centered on a central axis of the wheel bottom portion.

[0026] In a gear box of one aspect of the present disclosure, when viewed from one axial side of the wheel bottom, the other side rib is arranged on a straight line connecting the center axis of the wheel bottom and the center point of the worm engagement cylinder on the other axial side of the worm housing portion.

[0027] In the gear box according to one aspect of the present disclosure, the end portion of the worm housing portion on the other axial side is also open, and the circumferential width dimension of the other side rib is larger than the circumferential width dimension of the one side rib.

[0028] In a gear box of one embodiment of the present invention, the end portion on the other axial side of the worm housing portion is also open, and the radially outer end portion of the rib on the other side is located on the side farther away from the wheel mating cylinder portion in a direction orthogonal to the axial direction of the worm housing portion than the radially outer end portion of the rib on one side.

[0029] An electric assist device of one embodiment of the present invention comprises: a worm wheel having gear teeth on its outer peripheral surface; a worm having worm teeth meshing with the gear teeth on its outer peripheral surface; a gear box that accommodates the worm wheel and the worm on the inside; and an electric motor that is supported by the gear box and drives the worm to rotate, and the gear box is composed of the gear box of one embodiment of the present invention.

[0030] The configuration of the present disclosure can be implemented by appropriately combining the above-mentioned various aspects within a range where no contradiction occurs.

[0031] Effects of the Invention

[0032] According to one aspect of the present disclosure, it is possible to provide a gear box capable of effectively suppressing deformation during use, and an electric assist device including the gear box. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is a partially cutaway side view of an electric power steering device according to a first example of an embodiment of the present disclosure.

[0034] Figure 2 yes Figure 1 A cross-sectional view of part A.

[0035] Figure 3 yes Figure 1 BB cross-sectional view.

[0036] Figure 4 This is a diagram of the gear box of the first example as viewed from the front side in the axial direction.

[0037] Figure 5 yes Figure 4 CC section view and DD section view.

[0038] Figure 6 yes Figure 5 EE cross-sectional view.

[0039] Figure 7 This is a second example of an embodiment of the present disclosure. Figure 4 .

[0040] Figure 8 This is a third example of the embodiment of the present disclosure. Figure 6 .

[0041] Fig. 9 This is a fourth example of the embodiment of the present disclosure. Figure 4 .

[0042] Fig.10 yes Fig. 9 FF section view and GG section view.

[0043] Fig.11 yes Fig.10 HH cross-sectional view.

[0044] Fig.12 A fifth example of the embodiment of the present disclosure is equivalent to Figure 3 .

[0045] Fig.13 It is a perspective view of the gear box of the fifth example.

[0046] Fig.14 is the equivalent of the fifth example Figure 4 .

[0047] Fig.15 This is a diagram of the gear box of the fifth example as viewed from the rear side in the axial direction.

[0048] Fig.16 This is a sixth example of the embodiment of the present disclosure. Figure 4 .

[0049] Fig.17 This is a seventh example of the embodiment of the present disclosure. Figure 4 . DETAILED DESCRIPTION

[0050] [First example]

[0051] use Figure 1 to Figure 6 The first example of the embodiment of the present disclosure is described. In addition, in this example, the case where the electric assist device of one scheme of the present disclosure is applied to a column-assisted electric power steering device is described. However, the electric assist device of the present disclosure can also be applied to a pinion-assisted or double-pinion electric power steering device.

[0052] like Figure 1 As shown, the electric power steering system 1 of this example includes a steering wheel 2 , a steering shaft 3 , a steering column 4 , a pair of universal joints 5 a and 5 b , an intermediate shaft 6 , a steering gear unit 7 , and an electric assist device 8 .

[0053] In the following description of the electric power steering device 1 , the front-rear direction refers to the front-rear direction of the vehicle, and the left-right direction refers to the width direction of the vehicle.

[0054] The steering wheel 2 is supported and fixed to the rear end of the steering shaft 3. The steering shaft 3 rotatably supports the inner side of the steering column 4 supported by the vehicle body. The front end of the steering shaft 3 is connected to the pinion shaft 9 of the steering gear unit 7 via the universal joint 5a on the rear side, the intermediate shaft 6, and the universal joint 5b on the front side. Therefore, when the driver rotates the steering wheel 2, the rotation of the steering wheel 2 is transmitted to the pinion shaft 9 via the steering shaft 3, the pair of universal joints 5a, 5b, and the intermediate shaft 6. The rotation of the pinion shaft 9 is converted into a linear motion of the rack shaft (not shown) of the steering gear unit 7 meshing with the pinion shaft 9. As a result, by pushing and pulling a pair of tie rods 10, a steering angle corresponding to the rotation operation amount of the steering wheel 2 is given to the left and right steering wheels.

[0055] The electric power steering device 1 of this example is provided with a tilt mechanism for adjusting the vertical position of the steering wheel 2 according to the physique and driving posture of the driver. Therefore, the electric assist device 8 fixed to the front end of the steering column 4 is supported relative to the vehicle body so as to be able to swing about the tilt axis 11 in the left and right direction. The vertical position of the steering wheel 2 is adjusted by swinging the steering shaft 3, the steering column 4, and the electric assist device 8 in the vertical direction about the tilt axis 11.

[0056] like Figure 2 as well as Figure 3 As shown, the electric assist device 8 includes: a worm wheel 12 having gear teeth 13 on its outer peripheral surface; a worm 14 having worm teeth 15 on its outer peripheral surface that mesh with the gear teeth 13; a gear box 16 that accommodates the worm wheel 12 and the worm 14 on the inside; and an electric motor 17 that is supported by the gear box 16 and drives the worm 14 to rotate.

[0057] like Figure 2 As shown, the electric power steering device 1 of this example further includes a sensor housing 18 fixed to the front end of the steering column 4. In this example, the gear box 16 arranged on the front side and the sensor housing 18 arranged on the rear side are connected in the front-rear direction by a plurality of bolts 20 to form a housing 19.

[0058] The electric power steering device 1 of this example further includes a torsion bar 21 and an output shaft 22. In this example, the front end of the steering shaft 3 is inserted into the inner side of the housing 19 and is connected to the output shaft 22 via the torsion bar 21. The front end of the output shaft 22 protrudes forward from the inner side of the housing 19 and is connected to the intermediate shaft 6 via the universal joint 5a on the rear side (see FIG. Figure 1 The output shaft 22 is rotatably supported relative to the housing 19 by ball bearings 23 and 24 at two locations separated in the axial direction.

[0059] The electric power steering device 1 of this example further includes a torque sensor 25 housed inside the sensor housing 18 and arranged around the output shaft 22. The torque sensor 25 detects the direction and magnitude of the torque applied from the steering wheel 2 to the steering shaft 3. The electric motor 17 drives the worm 14 to rotate based on the detection signal of the torque sensor 25, the vehicle speed signal output from the vehicle speed sensor assembled in the transmission, etc., thereby applying auxiliary power to the output shaft 22 via the worm wheel 12. As a result, the force required for the driver to rotate the steering wheel 2 is reduced.

[0060] Hereinafter, the gear box 16 , the worm 14 , the worm wheel 12 , and the electric motor 17 constituting the electric assist device 8 of this example will be further described.

[0061] like Figure 2 to Figure 4 As shown in FIG. 1 , the gear box 16 includes a wheel housing portion 26 for housing the worm wheel 12 inside and a worm housing portion 27 for housing the worm 14 inside. In this example, the gear box 16 is integrally formed of a thermoplastic resin or a light alloy such as an aluminum alloy. That is, the connection between the various parts constituting the gear box 16 of this example described below refers to an integral connection, in other words, a physically continuous connection. The gear box 16 can be manufactured, for example, by injection molding of a thermoplastic resin or die casting of a light alloy such as an aluminum alloy.

[0062] In this example, the wheel housing portion 26 includes: a cylindrical wheel barrel portion 28, which is arranged around the worm wheel 12; an annular wheel bottom portion 29, which extends radially inward from the end portion of the wheel barrel portion 28, which is the front side of one axial side of the wheel housing portion 26; and a substantially cylindrical wheel interlocking barrel portion 30, which is connected to the radial inner end portion of the wheel bottom portion 29. In addition, in the following description of the wheel housing portion 26 and its components, the axial direction refers to the axial direction of the wheel housing portion 26 and its components. In this example, the radial inner end portion of the wheel bottom portion 29 is connected to the axial middle portion of the wheel interlocking barrel portion 30. In this example, the annular rib 31 is formed by a portion of the wheel interlocking barrel portion 30 that protrudes forward from the side surface of the front side of the wheel bottom portion 29. In this example, when the gear box 16 and the sensor housing 18 are combined to form the housing 19, the front end of the sensor housing 18 is embedded in the rear end of the wheel cylinder 28, and the front end surface of the sensor housing 18 abuts against the rear end surface of the wheel cylinder 28.

[0063] In this example, the electric motor 17 is mounted on the worm housing portion 27, and one axial side ( Figure 3 as well as Figure 4 The end of the right side in the middle is open, and the other side of the axial direction ( Figure 3 as well as Figure 4The worm housing portion 27 is a cylindrical shape with a bottom whose end (on the left side) is blocked. In addition, in the following description of the worm housing portion 27 and its components, the axial direction refers to the axial direction of the worm housing portion 27 and its components. However, in the case of implementing the structure of the present disclosure, the worm housing portion can also be constructed as a cylindrical shape with the end on one axial side and the end on the other axial side respectively opened, and the opening of the end on the other axial side is blocked by a cover body. The worm housing portion 27 is connected to a part of the circumferential direction of the radially outer end of the wheel housing portion 26. The central axis of the worm housing portion 27 is arranged in a twisted position relative to the central axis of the wheel housing portion 26. The internal space of the worm housing portion 27 is connected to the internal space of the wheel housing portion 26.

[0064] In this example, the worm housing portion 27 is provided at two locations separated in the axial direction, specifically, at the ends on both sides of the axial direction, more specifically, at Figure 3 The part with diagonal grid and Figure 4 The portion surrounded by the dotted line has roughly cylindrical worm-engaging barrel portions 32 and 33. The worm-engaging barrel portions 32 and 33 are respectively embedded portions for supporting the worm bearing 14 relative to the worm housing portion 27 or the bearing seat for holding the worm bearing. In this example, the inner diameter dimension and the axial width dimension of the worm-engaging barrel portion 32 on one axial side are larger than the inner diameter dimension and the axial width dimension of the worm-engaging barrel portion 33 on the other axial side. In this example, the worm housing portion 27 arbitrarily and additionally has a motor mounting flange 34 extending radially outward at the end portion on one axial side. The motor mounting flange 34 is a portion for mounting the electric motor 17, and has the function of increasing the rigidity of the peripheral portion of the electric motor 17 in the gear box 16.

[0065] In this example, the gear box 16 is further optionally and additionally provided with a hinge portion 35. The hinge portion 35 protrudes toward the front side from a portion of the radially outer end portion of the wheel housing portion 26 that deviates from the worm housing portion 27 in the circumferential direction. The hinge portion 35 has a hinge portion for inserting the tilt shaft 11 (see Figure 1 ) through hole 36. In this example, Figure 4 As shown, the hinge portion 35 is arranged at a position close to one side in the axial direction of the wheel housing portion 26. When the structure of the present disclosure is implemented, the position of the hinge portion 35 in the circumferential direction of the wheel housing portion 26 can be different from the present example as long as it deviates from the worm housing portion 27.

[0066] In this example, the gear box 16 is optionally and additionally provided with coupling flanges 58 protruding radially outward from two locations on the radially opposite sides of the wheel cylinder 28. Each coupling flange 58 has a threaded connection or a bolt 20 (see Figure 2) bolt holes 59, the bolts 20 are used to connect the gear box 16 and the sensor housing 18. In this example, one of the two connecting flanges 58 ( Figure 2 The coupling flange 58 (on the upper side) is arranged at the same circumferential position as the hinge portion 35. In addition, when the structure of the present disclosure is implemented, the number of coupling flanges and the phase of the arrangement in the circumferential direction can also be different from this example.

[0067] In this example, the gear case 16 further includes a side rib 37. The side rib 37 protrudes from the side surface on the front side of the wheel bottom 29 and the side surface on the front side of the worm housing 27, and connects the wheel fitting cylinder 30 and the worm fitting cylinder 32 on the axial side of the two worm fitting cylinders 32 and 33.

[0068] In the case of implementing the structure of the present disclosure, the protrusion amount of the side rib 37 to the front side can be set to any size. In this example, by adjusting the protrusion amount, as shown in FIG. Figure 5 As shown, the front end of the one-side rib 37 is located slightly rearward of the front end of the annular rib 31 .

[0069] In this example, if Figure 4 As shown, the rib 37 on one side extends in a radial direction centered on the central axis O of the wheel bottom 29. The central axis O of the wheel bottom 29 is also the central axis of the wheel housing portion 26. The radially inner end of the rib 37 on one side is connected to the outer peripheral surface of the annular rib 31. The radially outer end of the rib 37 on one side is connected to the side surface on the front side of the worm-engaging cylinder 32 on one axial side. That is, the radially outer end of the rib 37 on one side is firmly connected to the outer peripheral surface of the worm-engaging cylinder 32 on one axial side in an overlapping manner in the axial direction of the wheel bottom 29.

[0070] When the structure of the present disclosure is implemented, the extending direction of the one-side rib connecting the wheel fitting cylinder and the worm fitting cylinder on one axial side may be a direction different from the radial direction centered on the central axis of the wheel bottom.

[0071] When the structure of the present disclosure is implemented, the radially outer end of the one-side rib 37 can be located radially outward or radially inward compared with the case of this example if it is connected to the worm fitting cylindrical portion 32 on the one axial side.

[0072] In this example, if Figure 4 As shown, the one side rib 37 is arranged on a straight line α connecting the central axis O of the wheel bottom 29 and the center point P of the worm fitting cylinder 32 on one axial side when viewed from the front side of the wheel bottom 29. That is, in this example, the one side rib 37 is extended in the direction of the straight line α.

[0073] When the structure of the present disclosure is implemented, when the extending direction of one side rib is set as a radial direction centered on the central axis of the wheel bottom, the extending direction can be set as a direction offset by an arbitrary angle with respect to the direction of the straight line α.

[0074] In this example, if Figure 6 As shown, the cross-sectional shape of one side rib 37, specifically, the cross-sectional shape when cut by an imaginary plane perpendicular to the extension direction, is a solid rectangular shape. However, when implementing the structure of the present disclosure, the cross-sectional shape can be any shape.

[0075] In this example, the gear case 16 further includes another side rib 38. The other side rib 38 protrudes from the side surface of the front side of the wheel bottom 29 and the side surface of the front side of the worm housing 27, and connects the wheel fitting cylinder 30 and the worm fitting cylinder 33 on the other side of the two worm fitting cylinders 32 and 33 in the axial direction.

[0076] In the case of implementing the structure of the present disclosure, the protrusion amount of the other side rib 38 to the front side can be set to any size. In this example, by adjusting the protrusion amount, such as Figure 5 As shown, the front end of the other side rib 38 is located slightly rearward of the front end of the annular rib 31 .

[0077] In this example, if Figure 4 As shown, the other side rib 38 extends in a radial direction centered on the central axis O of the wheel bottom 29. The radial inner end of the other side rib 38 is connected to the outer peripheral surface of the annular rib 31. The radial outer end of the other side rib 38 is connected to the side surface of the front side of the worm fitting cylinder 33 on the axial other side. That is, the radial outer end of the other side rib 38 is firmly connected to the outer peripheral surface of the worm fitting cylinder 33 on the axial other side in an overlapping manner in the axial direction of the wheel bottom 29.

[0078] When the structure of the present disclosure is implemented, the extending direction of the other side rib connecting the wheel fitting cylinder and the worm fitting cylinder on the other axial side can also be set to a direction different from the radial direction centered on the central axis of the wheel bottom.

[0079] When the structure of the present disclosure is implemented, the radially outer end of the other-side rib 38 can be located radially outward or radially inward compared with the case of this example if it is connected to the worm fitting cylinder portion 33 on the other axial side.

[0080] In this example, if Figure 4As shown, the other side rib 38 is arranged on a straight line β connecting the center axis O of the wheel bottom 29 and the center point Q of the worm fitting cylinder 33 on the other axial side when viewed from the front side of the wheel bottom 29. That is, in this example, the other side rib 38 extends in the direction of the straight line β.

[0081] When the structure of the present disclosure is implemented, when the extension direction of the other side rib is set as a radial direction centered on the central axis of the wheel bottom, the extension direction can be set as a direction offset by an arbitrary angle with respect to the direction of the straight line β.

[0082] In this example, if Figure 6 As shown, the cross-sectional shape of the other side rib 38, specifically, the cross-sectional shape after being cut by an imaginary plane orthogonal to the elongation direction, is a solid rectangular shape. However, in the case of implementing the structure disclosed in the present invention, the cross-sectional shape can be set to any shape. In this example, the axial height and circumferential width between the one side rib 37 and the other side rib 38 are made substantially equal to each other, but in the case of implementing the structure disclosed in the present invention, the axial height and circumferential width of the one side rib can also be made larger than the other side rib.

[0083] In this example, if Figure 3 As shown, the worm 14 has worm teeth 15 meshing with the gear teeth 13 on the outer peripheral surface of the axial middle portion, and has internal splines 39 on the inner peripheral surface of the end portion on one axial side. The worm 14 is rotatably supported by ball bearings 40, 41, which are worm bearings, inside the worm housing portion 27.

[0084] The ball bearing 40 includes an outer ring 42, an inner ring 43, and a plurality of balls 44. The outer ring 42 is embedded in the worm fitting cylinder 32 on one axial side of the worm housing portion 27. The inner ring 43 is externally embedded in the end portion on one axial side of the worm 14. When the structure of the present disclosure is implemented, as an alternative, the outer ring 42 can be embedded in an annular bearing seat, the ball bearing 40 can be held by the bearing seat, and the bearing seat can be embedded in the worm fitting cylinder 32 on one axial side.

[0085] The ball bearing 41 includes an outer ring 45, an inner ring 46, and a plurality of balls 47. The ball bearing 41 is held by the bearing seat 48 by the outer ring 45 being fitted in the annular bearing seat 48. The bearing seat 48 is fitted in the worm fitting cylinder 33 on the other axial side of the worm housing portion 27. The inner ring 46 is fitted on the other axial side end of the worm 14. In the case of implementing the structure of the present disclosure, the outer ring 45 can be directly fitted in the worm fitting cylinder 33 on the other axial side instead.

[0086] like Figure 2 and Figure 3As shown, the worm wheel 12 has gear teeth 13 on the outer peripheral surface and is rotatably supported inside the wheel housing portion 26. Therefore, in this example, the worm wheel 12 is externally fitted and fixed to a portion of the output shaft 22 located between the two ball bearings 23 and 24.

[0087] The ball bearing 23 as a wheel bearing includes an outer ring 49, an inner ring 50, and a plurality of balls 51. The outer ring 49 is fitted inside the wheel fitting cylinder 30 of the wheel housing 26. The inner ring 50 is fitted outside the output shaft 22 at a portion adjacent to the front side of the worm wheel 12.

[0088] The ball bearing 24 includes an outer ring 52, an inner ring 53, and a plurality of balls 54. The outer ring 52 is fitted inside the front end of the sensor housing 18. The inner ring 53 is fitted outside the portion of the output shaft 22 adjacent to the rear side of the worm wheel 12.

[0089] In addition, in this example, the central axis of the worm 14 and the central axis of the worm wheel 12 are mutually orthogonal when viewed from a direction orthogonal to both the central axis of the worm 14 and the central axis of the worm wheel 12. That is, in the gear case 16, the central axis of the worm housing portion 27 and the central axis of the wheel housing portion 26 are mutually orthogonal when viewed from a direction orthogonal to both the central axis of the worm housing portion 27 and the central axis of the wheel housing portion 26. However, in the case of implementing the structure of the present disclosure, it is also possible to adopt a structure in which the central axis of the worm 14 and the central axis of the worm wheel 12 intersect at an acute angle when viewed from a direction orthogonal to both the central axis of the worm 14 and the central axis of the worm wheel 12. That is, it is also possible to adopt a structure in which the central axis of the worm housing portion 27 and the central axis of the wheel housing portion 26 intersect at an acute angle when viewed from a direction orthogonal to both the central axis of the worm housing portion 27 and the central axis of the wheel housing portion 26 in the gear case 16.

[0090] In this example, if Figure 3 As shown, the electric motor 17 is fixedly coupled to the motor mounting flange 34 of the worm housing 27 by bolts (not shown). In this example, the electric motor 17 has an external spline 56 on the outer peripheral surface of the front end portion of the output shaft 55. By engaging the external spline 56 with the internal spline 39 of the worm 14, the output shaft 55 and the worm 14 are connected in a torque-transmitting manner. In the case of implementing the structure of the present disclosure, the method of connecting the output shaft and the worm in a torque-transmitting manner is not particularly limited. For example, the output shaft and the worm can also be connected in a torque-transmitting manner via a coupling that can absorb (allow) mutual axial center deviation.

[0091] According to the electric assist device 8 of this example, deformation of the gear box during use can be effectively suppressed.

[0092] That is, in the electric assist device 8 of this example, when the assist power is generated, the meshing reaction force is applied to the worm 14 and the worm wheel 12 from the meshing portion between the worm teeth 15 and the wheel teeth 13. And these meshing reaction forces are transmitted to the worm housing portion 27 via the ball bearings 40 and 41, and are transmitted to the wheel housing portion 26 via the ball bearings 23. As a result, the gear box 16 is applied with a force F1 in a direction to separate the worm housing portion 27 and the wheel housing portion 26 from each other and a force F2 in a direction to twist the worm housing portion 27 and the wheel housing portion 26 (see FIG. 1 ). Figure 4 ). In addition, the force F2 in the direction of mutual twisting is generated based on the fact that at least the worm tooth 15 has a lead angle among the worm tooth 15 and the gear tooth 13. In addition, the direction of the force F2 in the direction of mutual twisting is reversed according to the rotation direction of the worm 14.

[0093] Furthermore, due to the force F1 in the direction away from each other, the gear case 16 tends to elastically deform in a tensile manner in the direction of the force F1.

[0094] In addition, based on the force F2 in the direction of mutual twisting, the gear box 16 has Figure 4 In this case, the direction in which the gear box 16 is stretched (the direction of the approximately straight line α, the direction of the approximately straight line β) is switched according to the rotation direction of the worm 14. Figure 4 The direction in which the annular rib 31 and the worm gear on one side of the axial direction are connected by the fitting cylinder 32 is not necessarily the same as the direction of the straight line α. Figure 4 The direction in which the annular rib 31 and the worm fitting cylindrical portion 33 on the other axial side are connected is not necessarily the same as the direction of the straight line β.

[0095] In the structure of this example, the one side rib 37 is provided to connect the worm housing portion 27 and the wheel housing portion 26. Therefore, the one side rib 37 can suppress the gear case 16 from elastically deforming in a tensile manner in the direction of the force F1 due to the force F1 in the direction of separation.

[0096] In addition, a side rib 37 connects the wheel interlocking cylinder 30 and the worm interlocking cylinder 32 on one axial side. Here, the wheel interlocking cylinder 30 is a portion in the wheel housing portion 26 that has a larger axial wall thickness and higher rigidity than the surrounding portions. In addition, in the worm housing portion 27, the worm interlocking cylinder 32 on one axial side is a portion that ensures higher rigidity in order to embed the ball bearing 40, and / or a portion whose rigidity becomes higher by embedding the ball bearing 40. That is, the side rib 37 connects the wheel interlocking cylinder 30, a portion with high rigidity in the wheel housing portion 26, and the worm interlocking cylinder 32 on one axial side, a portion with high rigidity in the worm housing portion 27. In addition, the extension direction of the side rib 37 is in Figure 4 Therefore, the force F2 in the direction of the mutual twisting of the gear case 16 can be effectively suppressed by the one side rib 37 from elastically deforming in a tensile manner in the direction of the straight line α.

[0097] In addition, in the structure described in International Publication No. 2019 / 087945, the rib connecting the worm housing portion and the wheel housing portion is not connected to at least the wheel interlocking cylinder portion. In contrast, in the structure of this example, the wheel interlocking cylinder portion 30 and the worm interlocking cylinder portion 32 on one axial side are connected by a rib 37 on one side, so that the gear box 16 can be more effectively suppressed from elastically deforming in a tensile manner in a direction roughly along the straight line α.

[0098] In the structure of this example, the electric motor 17 as a weight is supported at the end of one axial side of the worm housing portion 27. When the electric motor 17 is driven, inevitable motor vibration is generated. In view of this, in the structure of this example, the worm interlocking cylinder 32 and the wheel interlocking cylinder 30 located on one axial side near the electric motor 17 are connected by a one-side rib 37. That is, the rigidity of the peripheral portion of the electric motor 17 in the gear box 16 can be improved by the one-side rib 37. Therefore, it is easy to ensure the supporting rigidity of the gear box 16 for the electric motor 17 and to ensure the suppression effect of the gear box 16 on the motor vibration.

[0099] In the structure of this example, the other side rib 38 is provided to connect the worm housing portion 27 and the wheel housing portion 26. Therefore, the other side rib 38 can suppress the gear case 16 from elastically deforming in a tensile manner in the direction of the force F1 due to the force F1 in the direction of separation.

[0100] In addition, the rib 38 on the other side connects the wheel interlocking cylinder 30 and the worm interlocking cylinder 33 on the other axial side. Here, the wheel interlocking cylinder 30 is a portion in the wheel housing portion 26 that has a larger axial wall thickness and higher rigidity than the surrounding portions. In addition, in the worm housing portion 27, the worm interlocking cylinder 33 on the other axial side is a portion that ensures higher rigidity in order to embed the bearing seat 48, and / or a portion whose rigidity is increased by embedding the bearing seat 48. That is, the rib 38 on the other side connects the wheel housing portion 26, i.e., the wheel interlocking cylinder 30, and the worm housing portion 27, i.e., the worm interlocking cylinder 33 on the other axial side, which is a portion with high rigidity. In addition, the extension direction of the rib 38 on the other side is Figure 4 Therefore, the force F2 in the direction of the mutual twisting of the gear case 16 can be effectively suppressed by the other side rib 38 from elastically deforming in a tensile manner in the direction of the straight line β.

[0101] Thus, in the structure of this example, the one-side rib 37 and the other-side rib 38 connect the wheel fitting cylinder 30 located in the radial center of the wheel housing portion 26 and the two worm fitting cylinders 32 and 33 separated in the axial direction of the worm housing portion 27, thereby ensuring the rigidity of the gear box 16. In particular, the one-side rib 37 is arranged at a position closer to the electric motor 17 than the other-side rib 38, and has important functions in the gear box 16 to ensure the rigidity for supporting the electric motor 17 as a heavy object and the rigidity for suppressing the motor vibration generated when the electric motor 17 is driven.

[0102] [Second example]

[0103] use Figure 7 A second example of the embodiment of the present disclosure will be described.

[0104] In this example, compared with the structure of the first example, the gear case 16a has only one side rib 37 as a rib connecting the worm housing portion 27 and the wheel housing portion 26. That is, in this example, the gear case 16a does not have the other side rib 38 (see Figure 4 ).

[0105] That is, in the structure of this example, only one side rib 37 is provided, and this side rib 37 has the important function of ensuring the rigidity for supporting the electric motor 17 as a heavy object and the rigidity for suppressing the motor vibration generated when the electric motor 17 is driven, thereby ensuring the rigidity of the part of the gear box 16a close to the electric motor 17, and omitting the other side rib 38 can suppress the weight of the gear box 16a. The other structures and effects are the same as those of the first example.

[0106] [Third example]

[0107] use Figure 8 A third example of the embodiment of the present disclosure will be described.

[0108] In this example, the back side of the one side rib 37a and the other side rib 38a in the gear case 16b has a groove 57 extending along the extension direction of these ribs 37a, 38a. In this example, these ribs 37a, 38a are configured to be hollow by using the groove 57. According to the structure of this example, the gear case 16b can be configured to be lighter than the structure of the first example. The other structures and effects are the same as those of the first example.

[0109] [Fourth example]

[0110] use Figures 9 to 11 A fourth example of the embodiment of the present disclosure will be described.

[0111] In this example, if Fig. 9As shown, the circumferential width of each of the one side rib 37b and the other side rib 38b is reduced as it goes radially outward. In the case of implementing the structure disclosed in the present invention, the circumferential width of each of the one side rib and the other side rib can be constant in the radial inner part and reduced in the radial outer part as it goes radially outward.

[0112] In this example, if Fig.11 As shown, the cross-sectional shape of each of the one-side rib 37b and the other-side rib 38b is a solid trapezoidal shape.

[0113] In this example, if Fig.10 As shown, the projection amount of each of the one side rib 37b and the other side rib 38b toward the front side is constant in the radial inner part, and becomes smaller in the radial outer part as it goes toward the radial outer side. In the case of implementing the structure disclosed in the present invention, the projection amount of each of the one side rib and the other side rib toward the front side can also be made to decrease as it goes toward the radial outer side as a whole.

[0114] In the structure of this example, for the one side rib 37b and the other side rib 38b, not only the overall radial length and circumferential width are adjusted, but also the radial change rate of the circumferential width, the ratio of the length of the lower base to the length of the upper base of the trapezoidal shape as the cross-sectional shape, and the radial change rate of the protrusion amount to the front side are adjusted, thereby adjusting the rigidity of the gear box 16c. The other structures and effects are the same as those of the first example.

[0115] [Fifth example]

[0116] use Figure 12 to Figure 15 A fifth example of the embodiment of the present disclosure will be described.

[0117] In the structure of this example, the end of the worm 14 on one axial side is supported by a ball bearing 40 relative to the worm fitting cylinder 32 on one axial side of the worm housing 27a so as to be rotatable and swingable. The end of the worm 14 on one axial side is connected to the front end of the output shaft 55a of the electric motor 17 using a coupling 60 so as to be able to transmit torque and swing. The ball bearing 41 externally fitted to the end of the worm 14 on the other axial side is capable of moving in the far and near direction ( Fig.12 The worm engaging cylinder portion 33 is embedded in the worm housing portion 27a on the other axial side in a manner that allows the worm to be displaced in the vertical direction (in the up and down directions).

[0118] The structure of this example includes an elastic urging member 61 such as a leaf spring assembled between the worm fitting cylinder 33 and the ball bearing 41 on the other axial side. The elastic urging member 61 elastically urges the worm 14 toward the worm wheel 12 via the ball bearing 41. Thus, by suppressing the backlash between the gear teeth 13 and the worm teeth 15, the occurrence of a rattling sound when the rotation direction of the worm 14 is reversed can be suppressed.

[0119] In the structure of this example, the worm housing portion 27a constituting the gear box 16d is configured as a cylindrical shape with the end portion on one axial side and the end portion on the other axial side respectively opened. In the structure of this example, the end portion on the other axial side of the worm housing portion 27a is open, so that the assembly work of the elastic urging member 61 can be easily performed through the opening. In the assembled state of the electric assist device 8, the opening portion on the other axial side of the worm housing portion 27a is closed by a cover body 62 assembled to the opening portion.

[0120] The worm housing portion 27a of this example has a structure in which the other axial end is open, and thus the rigidity of the other axial end is likely to be lower than that of the worm housing portion 27 of the first example in which the other axial end is not open.

[0121] Therefore, in this example, in order to ensure or supplement the rigidity of the end portion on the other axial side of the worm housing portion 27a, the reinforcement level of the other side rib 38c for the end portion on the other axial side of the worm housing portion 27a is higher than the reinforcement level of the one side rib 37c for the end portion on one axial side of the worm housing portion 27a.

[0122] Specifically, the circumferential width of the other side rib 38c is made larger than the circumferential width of the one side rib 37c. Thus, by making the rigidity of the other side rib 38c larger than the rigidity of the one side rib 37c, the rigidity of the end portion on the other axial side of the worm housing portion 27a can be efficiently ensured.

[0123] In addition, the radially outer end of the other side rib 38c is positioned in a direction (ie, in a direction perpendicular to the axial direction of the worm housing portion 27a) relative to the radially outer end of the one side rib 37c. Fig.14 The side ( Fig.14 In this way, by ensuring that the connection length of the radially outer end of the other side rib 38c with respect to the end of the axially other side of the worm housing portion 27a is greater than the connection length of the radially outer end of the one side rib 37c with respect to the end of the axially one side of the worm housing portion 27a, the rigidity of the end of the axially other side of the worm housing portion 27a can be efficiently ensured.

[0124] In the case of implementing the structure disclosed in the present invention, it is also possible to adopt only one of the following structures: a structure in which the circumferential width dimension of the other side rib 38c is larger than the circumferential width dimension of the one side rib 37c, and a structure in which the radially outer end of the other side rib 38c is located farther away from the side of the wheel mating cylinder 30 than the radially outer end of the one side rib 37c in a direction orthogonal to the axial direction of the worm housing portion 27a.

[0125] In this example, if Fig.13 As shown, the protrusion amount of each of the one side rib 37c and the other side rib 38c toward the front side is arbitrarily and additionally made to decrease as it goes toward the radial outside, in other words, to increase as it goes toward the radial inside. That is, in this example, the volume of each of the one side rib 37c and the other side rib 38c increases as it goes toward the radial inside. Therefore, when the gear box 16d is injection molded or die-cast, the fluidity of the material from the radial outside to the radial inside can be improved in the molding space of each of the one side rib 37c and the other side rib 38c.

[0126] In this example, the gear box 16d is optionally and additionally provided with an opposite side rib 63. The opposite side rib 63 protrudes from the side surface of the front side of the wheel bottom 29. Fig.14 As shown in FIG. 1 , in the wheel bottom 29, the opposite side rib 63 is arranged on the radially opposite side relative to the one side rib 37c. The opposite side rib 63 extends in a radial direction centered on the central axis O of the wheel bottom 29. Fig.14 As shown, when viewed from the front side of the wheel bottom 29, the opposite side rib 63 is arranged on the central axis αc of the one side rib 37c. That is, in this example, the opposite side rib 63 extends in the direction of the central axis αc, that is, in the same direction as the one side rib 37c. The radially inner end of the opposite side rib 63 is connected to the outer peripheral surface of the annular rib 31. The radially outer end of the opposite side rib 63 is located at the radially outer end of the wheel bottom 29. In this example, by providing such an opposite side rib 63, the rigidity of the wheel housing portion 26 can be efficiently ensured.

[0127] In this example, an extension rib 64 is optionally and additionally provided, which protrudes from the outer peripheral surface of the wheel cylinder 28 and extends from the radially outer end of the opposite rib 63 toward the rear side along the axial direction of the wheel cylinder 28. In this example, the rear end of the extension rib 64 is located at the rear end of the wheel cylinder 28. In this example, by providing such an extension rib 64, the rigidity of the wheel housing portion 26 can be efficiently ensured.

[0128] In this example, arbitrarily and additionally, a hinge portion 35 is provided in a portion of the wheel bottom portion 29 located on the radially opposite side relative to the other side rib 38c. Therefore, the rigidity of the portion located on the radially opposite side can be ensured by the hinge portion 35. In the case of implementing the structure of the present disclosure, in the case where the hinge portion exists in a circumferential position different from that in this example, the opposite side rib protruding from the side surface of the front side of the wheel bottom portion 29 can also be arranged on the radially opposite side relative to the other side rib 38c. In this case, an extended rib protruding from the outer peripheral surface of the wheel cylinder 28 and extending from the radially outer end of the opposite side rib toward the rear side along the axial direction of the wheel cylinder 28 can also be provided.

[0129] In this example, the gear case 16d is optionally and additionally provided with a first rib 65. The first rib 65 extends from the side ( Fig.14 The first rib 65 protrudes from the end portion thereof (below the rib 65) and extends in the axial direction of the worm housing portion 27a. In this example, the first rib 65 is arranged over the entire axial length of the worm housing portion 27a. The end portion of the first rib 65 on one axial side is connected to the side surface of the motor mounting flange 34 on the other axial side. In this example, by providing such a first rib 65, the rigidity of the worm housing portion 27a can be efficiently ensured.

[0130] In this example, the gear case 16d is arbitrarily and additionally provided with a second rib 66. The second rib 66 protrudes from the side surface on the front side of the wheel bottom 29 and the side surface on the front side of the worm housing portion 27a, and connects the wheel fitting cylinder 30 and the first rib 65. In this example, as viewed from the front side of the wheel bottom 29, the second rib 66 is arranged on a straight line γ that passes through the center axis O of the wheel bottom 29 and is orthogonal to the center axis of the worm housing portion 27a. The radially inner end of the second rib 66 is connected to the outer peripheral surface of the annular rib 31. The radially outer end of the second rib 66 is connected to the middle portion of the first rib 65 in the extension direction. In this example, by providing such a second rib 66, the rigidity of the gear case 16d can be efficiently ensured.

[0131] In this example, arbitrarily and additionally, the gear case 16d further includes a third rib 67. The third rib 67 protrudes from the end portion on the front side of the outer peripheral surface of the worm housing portion 27a, and extends along the axial direction of the worm housing portion 27a. In this example, the third rib 67 is arranged on one axial side portion of the worm housing portion 27a. The end portion on one axial side of the third rib 67 is connected to the side surface on the other axial side of the motor mounting flange 34. The end portion on the other axial side of the third rib 67 is located between the radially outer end portion of the rib 37c on one side and the radially outer end portion of the rib 38c on the other side in the axial direction of the worm housing portion 27a, and more specifically, between the radially outer end portion of the rib 37c on one side and the second rib 66. In addition, as Fig.14As shown in FIG. 1 , when viewed from the front side of the wheel bottom 29, the central axis αc of the side rib 37c intersects with the third rib 67. In this example, by providing such a third rib 67, the rigidity of the worm housing portion 27a can be effectively ensured. Fig.14 As shown, when viewed from the front side of the wheel bottom 29, the central axis δ of the third rib 67 intersects with the central axis βc of the other side rib 38c. Also, when viewed from the front side of the wheel bottom 29, the intersection of the central axis δ of the third rib 67 and the central axis βc of the other side rib 38c overlaps with the worm fitting cylinder 33. By adopting such a structure, the rigidity of the worm housing portion 27a can be more efficiently ensured. The other structures and effects are the same as those of the first example.

[0132] In this example, all the arbitrary and additional elements are provided, but when implementing the structure of the present disclosure, one or more of these arbitrary and additional elements can be selectively adopted.

[0133] [Sixth example]

[0134] use Fig.16 A sixth example of the embodiment of the present disclosure will be described.

[0135] In the gear box 16e of this example, opposite side ribs 63 are arranged at two locations on the radially opposite side of one side rib 37c in the side surface on the front side of the wheel bottom 29 and offset from the central axis αc of the one side rib 37c to both sides in the circumferential direction. In the example shown in the figure, the two opposite side ribs 63 are arranged asymmetrically with respect to the central axis αc, but the two opposite side ribs 63 can also be arranged symmetrically with respect to the central axis αc. In the case of this example, by providing these opposite side ribs 63, the rigidity of the wheel storage portion 26 can also be efficiently ensured. Other structures and effects are the same as those of the fifth example.

[0136] [Seventh example]

[0137] use Fig.17 A seventh example of the embodiment of the present disclosure will be described.

[0138] In the gear case 16f of this example, the hinge portion 35 is arranged at a circumferential position on the radially opposite side to the worm housing portion 27a.

[0139] In this example, the gear case 16f has fourth ribs 68, which protrude from the side surfaces on the front side of the wheel bottom 29 and are arranged at the same circumferential position as the bolt hole 59. Each fourth rib 68 extends in a radial direction centered on the central axis O of the wheel bottom 29. The radially inner end of each fourth rib 68 is connected to the outer peripheral surface of the annular rib 31. The radially outer end of each fourth rib 68 is located at the radially outer end of the wheel bottom 29. In this example, by providing these fourth ribs 68, the rigidity of the wheel housing portion 26 can be efficiently ensured. Other structures and effects are the same as those in the fifth example.

[0140] The configuration of the present disclosure can be implemented by appropriately combining the configurations of the above-described embodiments within a range where no contradiction occurs.

[0141] Explanation of symbols

[0142] 1—Electric power steering device, 2—Steering wheel, 3—Steering shaft, 4—Steering column, 5a, 5b—Universal joint, 6—Intermediate shaft, 7—Steering gear unit, 8—Electric auxiliary device, 9—Pinion shaft, 10—Tie rod, 11—Tilt shaft, 12—Worm wheel, 13—Gear teeth, 14—Worm, 15—Worm gear teeth, 16, 16a, 16b, 16c, 16d, 16e, 16f—Gear Wheel box, 17—electric motor, 18—sensor housing, 19—housing, 20—bolt, 21—torsion bar, 22—output shaft, 23—ball bearing, 24—ball bearing, 25—torque sensor, 26—wheel storage portion, 27, 27a—worm storage portion, 28—wheel cylinder, 29—wheel bottom, 30—wheel embedded cylinder, 31—annular rib, 32—worm embedded cylinder, 33—worm Rod fitting cylinder, 34—motor mounting flange, 35—hinge, 36—through hole, 37, 37a, 37b, 37c—side ribs, 38, 38a, 38b, 38c—side ribs, 39—inner spline, 40—ball bearing, 41—ball bearing, 42—outer ring, 43—inner ring, 44—ball, 45—outer ring, 46—inner ring, 47—ball, 48—bearing seat, 4 9—outer ring, 50—inner ring, 51—ball, 52—outer ring, 53—inner ring, 54—ball, 55, 55a—output shaft, 56—external spline, 57—groove, 58—joining flange, 59—bolt hole, 60—coupling, 61—elastic force-applying member, 62—cover body, 63—opposite side rib, 64—extended rib, 65—first rib, 66—second rib, 67—third rib, 68—fourth rib.

Claims

1. A gear box, characterized in that: The invention comprises: a wheel housing portion for housing a worm wheel inside thereof; and a worm housing portion for housing a worm inside thereof, The wheel housing portion comprises: a wheel cylinder portion, which is arranged around the worm wheel; an annular wheel bottom portion, which extends radially inward from an end portion on one axial side of the wheel housing portion in the wheel cylinder portion; and a wheel fitting cylinder portion, which is connected to the radial inner end portion of the wheel bottom portion and has a wheel bearing embedded therein for supporting the worm wheel. The worm housing portion is configured as a cylinder with an open end portion on at least one axial side of the worm housing portion, and its own central axis is arranged in a twisted position relative to the central axis of the wheel housing portion, and is connected to a portion of the circumferential direction of the radially outer end portion of the wheel housing portion. The worm housing portion has a worm fitting cylinder portion at two locations separated along the axial direction of the worm housing portion, and a worm bearing for supporting the worm or a bearing seat for holding the worm bearing is embedded in the worm fitting cylinder portion. A rib on one side protrudes from the side surface on one axial side of the wheel housing portion in the wheel bottom portion and connects the wheel fitting cylinder portion and the worm fitting cylinder portion on one axial side of the worm housing portion in the two worm fitting cylinder portions. Also available: a first rib protruding from an end portion of an outer peripheral surface of the worm housing portion on a side away from the wheel housing portion and arranged to extend in the axial direction of the worm housing portion over the entire axial length of the worm housing portion; as well as a second rib, the second rib protruding from a side surface on one axial side of the wheel bottom and a side surface on one axial side of the wheel housing portion in the worm housing portion, and connecting the wheel fitting cylinder portion and the first rib, A radially outer end portion of the second rib is connected to a middle portion of the first rib in the extension direction.

2. The gearbox according to claim 1, characterized in that: It is provided with another side rib which protrudes from the side surface on one axial side of the wheel housing portion in the wheel bottom portion and connects the wheel fitting cylinder portion and the worm fitting cylinder portion on the other axial side of the worm housing portion in the two locations.

3. The gearbox according to claim 1, characterized in that: The worm housing portion has a motor mounting flange extending radially outward at an end portion on one axial side of the worm housing portion. further comprising a third rib, the third rib protruding from an end portion on one axial side of the wheel housing portion in the outer peripheral surface of the worm housing portion and extending in the axial direction of the worm housing portion, The end portion of the third rib on one axial side is connected to the side surface of the motor mounting flange on the other axial side, and the end portion of the third rib on the other axial side is arranged between the radially outer end portion of the rib on the one side and the radially outer end portion of the rib on the other side in the axial direction of the worm housing portion. When viewed from one axial side of the wheel housing portion, a central axis of the one side rib intersects with the third rib.

4. An electric assist device, characterized in that: have: A worm gear having gear teeth on an outer peripheral surface; A worm having worm teeth on its outer peripheral surface that mesh with the gear teeth; a gear box that accommodates the worm wheel and the worm inside; and an electric motor supported by the gear box and driving the worm to rotate, The gear box is constituted by the gear box according to any one of claims 1 to 3.

Images